First and foremost: HTML counts as programming among the majority of the population, so thank you for backing my argument :)
With regard to XHTML, I use HTML-oriented major-modes for editing these as I've found them to have much better support for HTML-specific use-cases (valid element-names, valid attributes, etc), and I think this is fairly common among most web-developers.
And to be honest, these days I almost exclusively stick to HTML5 anyway, which has "thrown out" all the XMLness of XHTML with requirements to close tags and all that stuff.
In that regard (with a risk of being a bit opinionated), I think considering nxml-mode as a major-mode for general text-editing (and HTML-editing especially) is looking backwards.
As for Docbook or used other XML-based formats for documentation, I'll admit I haven't done that, but I can see how that can also be popular use-cases too.
So nxml-mode may have to appease to different use-cases.
In that case, me saying nxml-mode is mostly used for programming-related tasks, may be opinionated (and definitely not scientific by any standards).
On the other hand I can't see how it is more opinionated or less scientific than assuming the majority of XML-work to be non-programming-related. But that's seemingly OK?
I'd love to hear other opinions on the matter.